1. Field of the Invention
This invention relates to zoom lenses suited to photographic cameras or video cameras, and more particularly, to zoom lenses in which a different focal length from those of the usual range of variation of the magnification is obtainable by utilizing a supplementary lens system and a space in which the lens units for zooming axially move, while still preventing the total length of the entire system from increasing.
2. Description of the Related Art
Many methods of changing the focal length of a zoom lens from those of the usual range of variation of the magnification to another one have been proposed.
For example, in Japanese laid-open Patent Application No. Sho 57-42010, a so-called rear conversion method is proposed in which the zoom lens comprising a focusing lens, variator, compensator and relay lens is all moved axially forward, and a supplementary lens system of negative refractive power is inserted into the image side of its last lens surface from the rear of the lens barrel to extend the focal length of the entire system.
This method enables the telephoto ratio to become relatively small, but, there were problems that, for the purpose of attaching the supplementary lens system, it was necessary first to detach the lens barrel and then to insert the supplementary lens system, and that the 4-unit type zoom lens of long total length tended to further increase in the total length.
Also, in Japanese laid-open patent application No. Sho 55-32046, a so-called afocal conversion method is proposed that is to attach a supplementary lens system of infinite focal length in front of the zoom lens to extend or contract the focal lengths of the entire system.
This method enables the shortest or longest focal length of the zoom lens to be respectively decreased or increased relatively easily, but, because the supplementary lens system is attached in front of the zoom lens, the entirety of the lens system tended to increase in size.
These rear conversion method and afocal conversion method, because of involving a large elongation of the total length of the lens or a great increase of the size of the lens system, are not very favorable particularly to the video camera which required that the size of the camera whole be minimized.
Besides these, there is another method of varying the range of variation of the magnification by moving the lens unit which is stationary during zooming. However, this method had a tendency that the number of movable lens units increased and the mechanism of the zoom lens system became more complicated.
Also, there is another method wherein a space into which the supplementary lens system is to be inserted is provided in a portion of the lens system (relay lens) which is stationary during zooming. But, this method must preserve the lens space for insertion of the supplementary lens unit previously in the lens system. Therefore, similarly to the rear conversion method, the total length of the entire system tended to increase.
Also, what is capable of separating the photographic lens out of the camera body at the camera mount portion is relatively easy to insert another optical system in between the zoom lens and the camera body and even the performance is good, but the total length of the lens tended to become larger than when in the usual photographing state.